BigDog, a creation of Boston Dynamics, is the poster child for quadruped robots.
A large, bulky pack-bot, it is designed to go anywhere a human soldier can go,
walkingh across any terrain, with four firm feet. It can walk on almost any
surface, run on those same surfaces, recover from being whacked hard when standing
still or running, and carry half a tonne with every step whilst it does all
this.

Unsurprisingly, BigDog is a military robot, designed to aid squads of troops
in the battlefield. Its three feet in length, one foot wide, two and a half
feet in height, and weighs in at 240 lbs, plus equipment which can weigh up
to 340 lb itself..

BigDog's legs are articulated, well, like a dog's. They absorb the shock of
impact with the ground, then recycle that kinetic energy for the next step.
In profile, it almost resembles two pairs of human legs, facing one another,
with a large load between them.

The bot is capable of running at up to 4mph, which doesn't sound like much,
until you remember that it can sustain this 4mph or something close to it, on
loose sand, rock, snow, wading a stream, tangled vegetation or tarmac, and keep
it up whilst transitioning between them. Slopes do slow it down, of course but
it can still scrabble its way up a 35 degree incline, which is about the limit
for a walking human too.

BigDog

BigDog has about 50 sensors. Inertial sensors measure the attitude and acceleration
of the robot itself, responding in such real-time that a researcher, squarely
shoving the robot with his upper body strength, was unable to get it to tip
- the robot shot out a foot and compensated long before it was in danger. Even
if it does fall, the robot can pick itself up, eventually, as it is impossible
for it to rest on its back.

In addition to the inertial sensors, other sensors monitor BigDog's hydraulic
pressure, flow and temperature, engine speed and temperature, etc.

Joint sensors measure motion and force of the actuators working at the joints.
As a side effect, this gives BigDog a crude sense of proprioception - it knows
where its body parts are in relation to each other, at all times. Because of
this body-sense, a range of behaviours has been developed. Currently, these
include standing up, squatting down, crawling along, shifting one leg at a time,
walking like a dog, trot like a horse, for a cantering gait between walk and
run, run like a (slow) cheetah. Additionally, it can gallop with small bounds,
ideal for clearing a crevasse, although actual jumping - from one height to
another - is not yet possible.

The ultimate goal for BigDog is to develop a robot that can go anywhere people
and animals can go. To this end, the current robots are not far short. Whilst
BigDog models currently cannot jump, that problem too, is being worked on. Another
robot by the same company, LittleDog, has been under continuous development
by DARPA and a half dozen competing research labs, attempting to develop improved
locomotion algorithms and self-learning for use on BigDog.

Of course the one problem it does have, in current models, is the whine of
the diesel engine, kinda lets the enemy know its coming. Sneaking up is not
something BigDog has mastered, yet.